1. Field of the Invention
The present invention relates to a flow measuring device which is used in the technical field of metering such as in a gas meter, flow meter, and so forth, and, in particular, to a heat sensing type flow measuring device and flow measuring method which measures a flow velocity using a change of a resistance value of a heating resistor due to a temperature change resulting from fluid flowing.
2. Description of the Related Art
Recently, a flow meter using a fluidic flow control device is being studied as a next generation gas meter which will substitute for a conventional integrating-type film-type meter. However, the fluidic flow control device cannot measure a low flow rate equal to or less than 150 liters/hour (hereinafter, referred to as 'L/H'). Therefore, as a flow measuring device for a low flow rate range, a flow measuring device referred to as a flow sensor is used which is of a heat sensing type wherein a heating resistor is arranged on a micro bridge and is exposed to a fluid.
As a flow measuring device (flow sensor), various systems have been proposed.
Japanese Utility-Model Publication No.7-51618, Japanese Utility-Model Publication No. 7-117436, and Japanese Laid-Open Patent Application No. 5-312616 disclose flow measuring devices of a system (first system) in which a balanced bridge circuit is used and a heating resistor is driven to a fixed temperature. An example of such a flow measuring device in the prior art will now be described with reference to FIG. 1. The flow measuring device 1 uses a balanced bridge circuit 2. The balanced bridge circuit 2 has a resistance thermometer bulb 3 and a heating resistor 4. These resistors 3 and 4 have the same large temperature coefficient of resistance. A temperature setting resistor 5 which has a small temperature coefficient of resistance is connected to the resistance thermometer bulb 3. The resistors 3 and 5 form resistance thermometer bulb portion 6. Another resistor 7 is connected to the resistance thermometer bulb portion 6, and another resistor 8 is connected to the heating resistor 4. The resistors 7 and 8 have the same small temperature coefficient of resistance. As a result of the resistors 7 and 8 being connected, the balanced bridge circuit 2 is formed.
The point or node 9 at which the resistors 3 and 4 are connected to one another is grounded. To the point 10 at which the resistors 7 and 8 are connected to one another, a direct-current power source 12 is connected via a control transistor 11. A differential amplifier 15 is connected to the connection point 13 of the resistance thermometer bulb portion 6 and resistor 7 and the connection point 14 of the resistors 4 and 8. The differential amplifier 15 is connected to the control transistor 11 so as to provide feedback.
In the flow measuring device 1, the resistance thermometer bulb 3 and the heating resistor 4 are placed in a flow path, and feedback control is performed so that the balance of the balanced bridge circuit 2 is maintained. In the state in which the balanced bridge circuit 2 balances, power consumed by the heating resistor corresponds to the flow rate of the fluid. Therefore, the flow rate of the fluid can be measured by measuring the output of the differential amplifier 15 and the voltage of the connection point 14. According to the flow measuring device 1, feedback control is performed on the balance of the balanced bridge circuit 2 which changes according to change of the flow rate of the fluid. Thereby, the flow rate of the fluid can be measured. However, in the flow measuring device 1, when the temperature of the fluid changes, an error occurs in the measurement result. Therefore, it is necessary to correct the flow measurement result according to change of the temperature of the fluid.
For this purpose, in a flow measuring device disclosed in Japanese Laid-Open Patent Application No. 4-204119, a resistance thermometer bulb is detachable from a balanced bridge circuit. The resistance thermometer bulb detached from the balanced bridge circuit is driven by a constant current and the voltage drop is measured. From the measurement result, the temperature of the fluid is measured. In a flow measuring device disclosed in Japanese Laid-Open Patent Application No. 5-164583, a heating resistor which is driven by a constant voltage and a resistance thermometer bulb which is driven by a constant current are provided separately. The flow-rate measurement error is corrected as a result of multiplying flow rate data obtained from the heating resistor by fluid temperature data obtained from the resistance thermometer bulb. In a flow measuring device disclosed in Japanese Laid-Open Patent Application No. 2-120621, a resistance thermometer bulb of a pair of resistance thermometer bulbs is placed upstream of a heating resistor and another resistance thermometer bulb of the pair of resistance thermometer bulbs is placed downstream of the heating resistor. A balanced bridge circuit is formed including these resistance thermometer bulbs. Data corresponding to the temperature difference of the pair of resistance thermometer bulbs is detected from a balance change of the balanced bridge circuit. Data corresponding to the heating temperature of the heating resistor is detected from the resistance values of the pair of resistance thermometer bulbs. By dividing the temperature difference data by the heating temperature data, data corresponding to the fluid flow rate is obtained.
A second system is provided by a constant-current driving type system, in which a heating resistor is placed upstream of a flow path and another heating resistor is placed downstream of the flow path. Each of the heating resistors is heated by a constant current. The flow rate of the fluid is measured based on change of the resistance values of the heating resistors.
An example of such a flow measuring device in the prior art disclosed in Japanese Patent Publication No. 3-52028, for example, will now be described with reference to FIG. 2. A flow measuring device 21 which will now be described has a first heating resistor 22 and a second heating resistor 23, each of which has a large temperature coefficient of resistance. These heating resistors 22 and 23 are formed, for example, by printed wiring on a surface of a wiring substrate (not shown in the figure). To the first and second heating resistors 22 and 23, first and second operational amplifiers 24 and 25, which act as first and second measuring means, are connected, respectively. Thereby, feedback loops 26 and 27 are formed.
A potentiometer 29 is connected to a pair of input terminals 28. To the potentiometer 29, the feedback loops 26 and 27 are connected via resistors 30 and 31. Thereby, a portion from the input terminals 28 to the feedback loops 26 and 27 acts as first and second power supply means which supply variable-voltage and constant-current power to the first and second heating resistors 22 and 23. To the feedback loops 26 and 26, resistors 32, 33, 34 and 35 and a third operational amplifier 36 are connected. The third operational amplifier 36 and a ground terminal 37 are connected to a pair of output terminals 38, respectively.
The flow measuring device 21 can measure the flow rate of a fluid such as gas. In this case, the wiring substrate is appropriately placed in the flow path of the fluid. Thereby, the first and second heating resistors 22 and 23 are positioned in the stated order in the fluid flow direction. When power is supplied to the feedback loops 26 and 27 via the input terminals 28 in this state, variable-voltage and constant-current power is supplied to the first and second heating resistors which are then heated.
When the fluid flows in this state, a heat quantity moves from the upstream first heating resistor 22 to the downstream second heating resistor 23. Thereby, the resistance value of the first heating resistor 22 which has a large temperature coefficient of resistance decreases, and the resistance value of the second heating resistor 23 which also has a large temperature coefficient of resistance increases. However, the current values of the feedback loops 26 and 27 are maintained constant, respectively. Thereby, the output voltage of the feedback loop 26 decreases and the output voltage of the feedback loop 27 increases. This voltage difference is detected by the third operational amplifier 36. The thus-detected voltage difference corresponds to the flow-rate of the fluid, and thereby, the flow rate of the fluid can be detected from the output voltage of the output terminals 38.
Another example of a signal processing system of such a constant-current driving type flow measuring device is disclosed in Japanese Patent Publication No. 6-64080 and will now be described with reference to FIG. 3. FIG. 3 shows an electric arrangement of the signal processing system. In a flow measuring device 41, two temperature sensing resistors (heating resistors) 42 and 43 are positioned in a flow path of a fluid, and are connected in series. Further, a temperature sensing resistor 44 which is used for measuring the temperature around the sensor is provided. A current source 45 is connected to the temperature sensing resistor 44. Further, an operational amplifier 46 is provided. The plus input terminal of the operational amplifier 46 is connected to the connection point P.sub.11 of the temperature sensing resistor 44 and current source 45. The output terminal of the operational amplifier 46 is connected to one end of the series-connected resistor series (one end of the temperature sensing resistor 42). An output terminal is drawn from the connection point P.sub.12 of the temperature sensing resistor 42 and 43.
In the arrangement, the output voltage of the operational amplifier 46 is supplied to the resistor series, and the flow rate is measured by using a voltage change at the connection point P.sub.12. The temperature sensing resistor 44 is used for measuring an ambient temperature and the voltage at the connection point P.sub.11 changes according to change of the ambient temperature. Thus, influence of the ambient temperature, otherwise on the output voltage obtained at the connection point P.sub.12, is eliminated.
Another example is disclosed in Japanese Laid-Open Patent Application No. 8-509066. FIG. 4 shows an electronic arrangement thereof. In the example of a flow measuring device 51, four resistors 53, 54, 55 and 56 form a balanced bridge circuit 52. The resistors 55 and 56 are ordinary resistors and have substantially equal characteristics. The resistors 53 and 54 are placed in a flow path 57 of a fluid, and are heating resistors which heat as a result of receiving a power supply and have substantially equal characteristics. In this case, the resistor 53 is placed on the upstream side. A current source 58 is connected to the connection point of the resistors 54 and 56 of the balanced bridge circuit 52. A transistor 62 is connected to the connection point of the resistors 53 and 55. Further, the connection point A of the resistors 53 and 54 is connected to the minus input terminal of an operational amplifier 59. The plus input terminal of the operational amplifier 59 is grounded. A feedback resistor 60 is connected between the minus input terminal and the output terminal of the operational amplifier 59. Thereby, the connection point A is artificially grounded through the operational amplifier 59. The connection point B of the resistors 55 and 56 is connected to the minus input terminal of an operational amplifier 61. The plus input terminal of the operational amplifier 61 is grounded, and the resistor 55 and transistor 62 are connected between the minus input terminal and the output terminal of the operational amplifier 61.
In this arrangement, due to flow of the fluid in the flow path 57, the balanced state is disrupted or broken. However, in order to maintain the artificial grounding of the connection point A, the current quantity flowing to the connection point A changes. The change of the current of the connection point A is obtained as the output of the operational amplifier 59. Therefore, the flow rate of the fluid can be measured by monitoring the output of the operational amplifier 59.
In such a type of a flow measuring device, as described above, when the temperature of a fluid (ambient temperature) changes, an error occurs in the flow measurement result. Therefore, it is necessary to correct the flow measurement result according to change of the fluid temperature. A temperature sensing type flow measuring device has a measuring capability for a low flow rate range. For the temperature sensing type flow measuring device, it is important to accurately measure the state in which the fluid does not flow and thus the flow rate is zero. However, the state corresponding to the zero flow rate changes according to change of the ambient temperature. In order to improve measurement accuracy, it is necessary to provide for flow measurement stability independent of temperature effects in consideration of the zero flow rate state.
However, in a flow measuring device in the system disclosed in the Japanese Laid-Open Patent Application No. 4-204119, although the flow rate and temperature of a fluid are measured, no function is provided for correcting the flow measurement result based on the temperature. In a flow measuring device in the system disclosed in Japanese Laid-Open Patent Application No. 5-164583, when a heating resistor is degraded over a period of time and thereby flow rate data changes, it is not possible to correct this error and obtain an accurate flow rate. With regard to this point, in a flow measuring device in the system disclosed in Japanese Laid-Open Patent Application No. 2-120621, even if a heating resistor is degraded due to time, flow measurement can be performed independent of the degradation. However, a resistance thermometer bulb should be heated by a heater, and, during a long period of use, an error occurs in measurement due to degradation of the resistance thermometer bulb. Further, as a result of power supply to a balanced bridge circuit, the resistance thermometer bulb heats, and the heating temperature changes according to the ambient temperature and flow rate. Accordingly, an error occurs in the measurement result based on the resistance value of the resistance thermometer bulb.
Consequently, in these flow measuring devices, each uses a balanced bridge circuit and thus measures flow rates. In this system, it is necessary that the balance of the balanced bridge circuit is maintained. However, the plurality of resistors forming the balanced bridge circuit are degraded over time, and, in particular for the heating resistor, the degree of the degradation varies with heating temperatures and environments of use. Therefore, it is difficult to maintain the balance of the balanced bridge for a long period. Further, when the balanced bridge is used, in comparison to the voltage applied to the balanced bridge, the voltage applied to each heating resistor should be low. Therefore, for example, a battery-driven flow measuring device is provided in which a voltage range is narrow, and only very low voltage is applied to each heating resistor. As a result, the selectable range of the resistance value of each heating resistor is narrow, or, it is necessary to select a resistance value which is difficult to implement in a manufacturing method.
In the flow measuring device 21 shown in FIG. 2 and disclosed in Japanese Patent Publication No. 3-52028, the first and second heating resistors 22 and 23, placed in the stated order in the fluid flowing direction, are driven by constant currents, and the flow rate is measured from the difference between the two end voltages. However, in this system, the measuring result includes not only the flow rate of the fluid but also the temperature of the fluid. Accordingly, it is not possible to accurately measure the fluid flow rate.
In fact, the resistance value "R" of a heating resistor is approximated so that "R=R0(1+.alpha..multidot.dT)", where "R0" represents the resistance value of the heating resistor in a reference condition where the flow rate is "0" in a predetermined temperature; "dT" represents the difference between the reference temperature and the temperature of the heating resistor and ".alpha." represents the temperature coefficient of resistance of the heating resistor. Accordingly, the resistance value R of the heating resistor changes according to change of its temperature. Further, the temperature of the heating resistor in a fluid is obtained as a result of adding the heating temperature due to the Joule heat by a current flowing through the heating resistor and the temperature of the fluid, and subtracting, from the addition result, the temperature due to the heat quantity which moves to the fluid.
Thus, in the flow measuring device 21, the resistance value of each of the upstream and downstream first and second heating resistors 22 and 23 changes according to a change of the temperature of a fluid. Therefore, when constant currents flows therethrough, respectively, the Joule heats change and thereby heating temperatures change. The fluid removes part of the Joule heats from the heating resistors. When the Joule heats change, the heat quantities moving from the first and second heating resistors to the fluid change, respectively.
The first and second operational amplifiers 24 and 25 output voltages corresponding to the heating temperatures of the first and second heating resistors 22 and 23 which depend on the above-described effects. The third operational amplifier 36 outputs the difference of these voltages as the fluid flow measurement result. Accordingly, the output result includes the effect of the difference between the heat quantities which move from the first and second heating resistors to the fluid, respectively. Therefore, the output result changes according to change of the temperature of the fluid. Further, if the temperature coefficients of resistance and the resistance values at the reference temperature of the first and second heating resistors 22 and 23 are different, even if the fluid does not flow, the output result occurs when the temperature of the fluid changes. Accordingly, it is difficult to determine whether the fluid is flowing or not flowing (zero flow rate).
In the flow measuring device 31 shown in FIG. 3 and disclosed in Japanese Patent Publication No. 6-64080, it is necessary to adjust the resistance values of the temperature sensing resistors 42, 43 and 44 so that a specific condition is fulfilled for the purpose of removing ambient temperature effects for the temperature sensing resistors 42, 43 and 44. However, manufacturing of the temperature sensing resistors having the adjusted resistance values is difficult. In the flow measuring device 51 shown in FIG. 4 and disclosed in Japanese Laid-Open Patent Application No. 8-509066, the resistors 53 and 54 of the resistors forming the balanced bridge circuit 52 are likely to be degraded because of the heating of the resistors 53 and 54. Therefore, it is difficult to maintain the balance state of the balanced bridge circuit 52 for a long period. Accordingly, such a flow measuring device is not suitable for a gas meter.